1. Field of the Invention
The invention relates to organosol polyester coating materials and to a process for their preparation. Coating compositions of this kind are suitable for the thick-layer coating of metallic substrates.
2. Description of the Background
The coating of metallic substrates on industrial strip coating units employs coating materials based on various binder systems. The principal systems are formulated on the basis of polyesters, polyester-polyurethane, PVC (plastisols, organosols) or polyvinylidene fluoride (PVDF). In addition, systems based on silicone-modified polyesters and on epoxy resin or acrylic resin are also employed.
The coating materials, which can be processed in liquid form at room temperature, are applied to strips (steel, zinc-plated steel, aluminum, etc.) on industrial-coating units, generally with roller coating units. This application method has a number of advantages. On typical high-performance units in the coil-coating industry it permits high belt speeds (up to 200 m/min) and ensures highly uniform application of the coating. Film formation and curing of the liquid coating materials on belts takes place in through-type air dryers. The prior art coating materials mentioned can in this case be processed in one dryer pass only with film thicknesses of up to about 30 micrometers (.mu.m). An exception to this are PVC-based (plastisol) coatings, which can be processed with coat thicknesses in the range 80-200 .mu.m and, at the same time, at relatively high belt speeds to give thick-layer coatings.
The precoated strips are cut and shaped to produce a large number of industrial products, for example, for the construction industry, in appliance construction, in transport or for the metal packaging industry (can coating). Thick-layer coatings are of particular interest when the product produced from precoated metal is required to have high resistance to corrosive attack, for example in exterior architecture or in interior architecture or when appliances are operated in a humid climate. Thick-layer coatings based on PVC plastisols are notable for outstanding corrosion protection, very good formability, easy selection of color, and ready processability on the coating units. On the other hand, coating compositions of this kind based on PVC have certain disadvantages. Because of very poor adhesion properties on metallic substrates, PVC plastisols can only be processed in two coats on specific adhesion primers (for PVC plastisols). The plasticizers present in typical PVC plastisols are strong swelling agents, so that the selection of the primer is restricted to specific systems.
A critical disadvantage of PVC thick-layer coatings, however, is that the disposal of PVC-coated metal sheets is becoming increasingly problematic and costly.
This is also true for the process of coating metallic substrates on strip coating units by laminar application of PVC films (40-400 .mu.m film thickness). Although films based on halogen-free polymer (polyolefins, polyacrylate etc.) are available, certain disadvantages are known. For instance, special laminating adhesives are required. Creasing with relatively thin films, electrostatic dust attraction, fluctuations in gloss, and loss of film embossing are known problems of the film lamination process.
Also known is the process of hotmelt coating of metal strips with thermoplastic polymers. A disadvantage of this process is the high mechanical expenditure required for extrusion coating.
DE 40 12 020 describes the thick-layer coating of strips with powder coatings based, for example, on polyester. A disadvantage of this process is the low belt speed, which is limited by the electrostatic application process for the powder coatings.
EP 0 466 310 describes the use of solvent-insoluble crystalline polymers in dispersions having particle sizes which are preferably in the range 1-10 .mu.m as film-forming constituents for coatings. For the stability of such dispersion coating materials (emulsion paints) specific stabilizers are described as indispensable. According to the statements in this patent document it is not possible to prepare dispersion coating materials using fine powders of crystalline, thermoplastic polymers produced by physical grinding. The reason given for this is that coating materials with an extremely nonhomogeneous structure and, accordingly, poor film properties are the result of this process.
The object on which the invention is based is to develop halogen-free, heat-curing coating compositions for the strip coating process which can be applied as a liquid at room temperature, for example, by the roller coating process, and which can be processed at very high belt speeds to give thick-layer coatings having good mechanical deformability and very good corrosion protection.